Flexible circuit board having electrical resistance heater trace

ABSTRACT

A flexible circuit board ( 20 ) having a substantially rigid substrate ( 22 ) and an electrical resistance heater trace ( 24 ). The substantially rigid substrate ( 22 ) has a first portion ( 26 ), a second portion ( 28 ), and a bend region ( 30 ). The bend region ( 30 ) interconnects the first portion ( 26 ) and the second portion ( 28 ). The electrical resistance heater trace ( 24 ) is formed on the bend region ( 30 ) of the substrate ( 22 ). The first portion ( 26 ) of the substrate ( 22 ) is capable of being folded relative to the second portion ( 28 ) of the substrate ( 22 ) to form at least one bend ( 72, 172 ) in the bend region ( 30 ) when an electric current is applied to the heater trace ( 24 ). There is also an electronic control unit ( 60, 160 ) that includes the flexible circuit board ( 20 ) and a method of assembling an electronic control unit ( 60, 160 ).

FIELD OF THE INVENTION

[0001] This invention in general relates to circuit boards forelectronic control units and, more particularly, to a method andapparatus for increasing the flexibility of a circuit board throughlocalized heating by an electrical resistance heater trace.

BACKGROUND OF THE INVENTION

[0002] Engine mounted electronic control units for vehicular engines aresubject to a high level of heat and vibration and generally disposed ina confined space. In these applications, electronic components andcircuits may be formed on a relatively thin substrate that is enclosedwithin a rigid housing. In order to enhance thermal performanceprotection against engine vibration, the thin substrate is affixed to arigidizer that may be bent to produce a reduced size module and thatalso functions as a heat spreader.

[0003] For example, one type of direct engine mount application uses aPolybent™ printed circuit board. This is a printed circuit board thathas a flexible circuit board that is mounted to an aluminum rigidizer.The rigidizer provides mechanical support for the printed circuit boardand assists in the dissipation of heat generated by components on theprinted circuit board, which heat is conductively transferred from thecomponents to the underlying rigidizer. One reference that describes anelectronic control unit using a Polybent™ printed circuit board is U.S.Pat. No. 5,998,738, which is owned by the assignees of the presentinvention and hereby incorporated by reference herein in its entirety.

[0004] As the functionality of electronic control units has increasedover time, the corresponding circuitry has become increasingly dense andcomplex. As a result, electronic control units have been migrating fromthe use of two-layer printed circuit boards to the use of four-layerprinted circuit boards. One result of four-layer printed circuit boardsis increased thickness. Thicker flexible circuit boards are known tocrack or split when bent, resulting in a control unit that must bediscarded. Discarded control units results in excessive manufacturingcosts and waste, especially since the printed circuit board must bepopulated with components before being folded.

[0005] Moreover, the type of material used for the substrate will affectthe degree of flexibility of the board. For example, a material that iswell known in the construction of circuit boards is a type of epoxyglass known as FR4. FR4 has a glass weave impregnated with epoxy resinand is generally known to be relatively stiff. Although FR4 and othermore rigid substrates are substantially less expensive than veryflexible substrates, the use of more rigid substrates presents the addedproblem of cracking and splitting when trying to bend the substrate to aconfined space.

[0006] U.S. Pat. No. 6,292,370, owned by the assignees of the presentinvention and hereby incorporated by reference herein in its entirety,describes that cracking and other damage may be avoided by heating thesubstrate to within about 10° C. of the glass transition temperature ofthe FR4 material. The reference recites that this may be accomplished bypassing the circuit substrate through an oven.

[0007] A need exists, however, for improved devices and methods forincreasing the flexibility of more rigid substrates to reduce splittingand cracking. For instance, a need exists to reduce the cost in theenergy expended in passing the circuit substrate through an oven. Thereis also a need to reduce the time needed to heat up the substrate overthe known method of passing the circuit substrate through an oven. Itis, therefore, desirable to provide an improved device and method ofheating a substrate to overcome most, if not all, of the precedingproblems.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 is a plan view of a flexible circuit board in oneembodiment of the present invention;

[0009]FIG. 2 is a plan view of the opposite side of the flexible circuitboard in FIG. 1;

[0010]FIG. 3 is a cross section view of a bend region of the flexiblecircuit board in FIG. 1;

[0011]FIG. 4A is an exploded perspective view of an electronic controlunit in accordance with an embodiment of the present invention;

[0012]FIG. 4B is a side view of a portion of the electronic control unitin FIG. 4A;

[0013]FIG. 5A is an exploded perspective view of an electronic controlunit in accordance with another embodiment of the present invention;

[0014]FIG. 5B is a side view of a portion of the electronic control unitin FIG. 5A; and

[0015]FIG. 6 is a flow diagram of one embodiment of a method forassembling an electronic control unit according to the presentinvention.

[0016] While the invention is susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and will be described in detail herein. However,it should be understood that the invention is not intended to be limitedto the particular forms disclosed. Rather, the invention is to cover allmodifications, equivalents and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

[0017] What is described is a device and method for increasing theflexibility of a circuit board through localized heating. For purposesof illustration, an example of the device and method will be describedin the context of an electronic control unit for a vehicle. However, thepresent invention is not limited to units for vehicles but may alsoapply to other housings or devices where flexibility of a circuit boardis needed.

[0018] To this end, generally, in one embodiment there is a flexiblecircuit board having a substantially rigid substrate and an electricalresistance heater trace. The substantially rigid substrate has a firstportion, a second portion, and a bend region. The bend regioninterconnects the first portion and the second portion. The electricalresistance heater trace is formed on the bend region of the substrate.The first portion of the substrate is capable of being folded relativeto the second portion of the substrate to form at least one bend in thebend region when an electric current is applied to the heater trace.

[0019] In another embodiment, there is an electronic control unit havinga rigidizer, a substantially rigid substrate, and an electricalresistance heater trace. The rigidizer has a first rigidizer portion anda second rigidizer portion that is interconnected by a connectingrigidizer portion. The substrate has a first portion, a second portion,and a bend region. The bend region interconnects the first portion andthe second portion. The first portion of the substrate is attached tothe first rigidizer portion. The second portion of the substrate isattached to the second rigidizer portion. The electrical resistanceheater trace is formed on the bend region of the substrate. The firstportion of the substrate is capable of being folded relative to thesecond portion of the substrate to form at least one bend in the bendregion when an electric current is applied to the heater trace.

[0020] Further, in another embodiment, there is a method for assemblingan electronic control unit that includes the steps of: providing asubstantially rigid substrate having a first portion and a secondportion interconnected by a bend region; forming an electricalresistance heater trace on the bend region of the substrate; providing arigidizer having a first rigidizer portion and a second rigidizerportion interconnected by a connecting rigidizer portion; attaching thefirst portion of the substrate to the first rigidizer portion; attachingthe second portion of the substrate to the second rigidizer portion;applying an electrical current to the electrical resistance heatertrace; and folding the first portion of the substrate relative to thesecond portion of the substrate to form at least one bend in the bendregion.

[0021] Now, turning to the drawings, an example use of a device andmethod will be explained in the context of an electronic control unitfor a vehicle. FIGS. 1 and 2 show opposite sides of a flexible circuitboard 20 that would reside within an electronic control unit. In oneembodiment, generally, the flexible circuit board has a substantiallyrigid substrate 22 and an electrical resistance heater trace 24. Thesubstantially rigid substrate 22 has a first portion 26 and a secondportion 28 that is interconnected by a bend region 30. The substantiallyrigid substrate 22 may be made of FR4 or other more rigid organic basedsubstrates. FR4 is a material having at least one layer of glass weavesimpregnated with epoxy resin. Other types of epoxy glass and polyimidesare also commercially available. The substantially rigid substrate 22may also be made of multiple layers of a substrate material laminatedtogether that make the substrate more stiff or brittle.

[0022] The electrical resistance heater trace 24 is formed on the bendregion 30 of the substrate 22. In one embodiment, as shown in FIG. 1,the electrical resistance heater trace 24 is formed in a serpentineshape that is uniform along the entire bend region 30 of the substrate22. In one suitable device, an adequate width of the heater trace 24 wasfound to be about 10 mils and the spacing between each serpentine tracewas about 25 mils, center-to-center. The heater trace 24 may be made ofa metallic conductive material such as copper, nickel or gold. In thisembodiment, each end of the electrical resistance heater trace 24 hasconnector traces 32 and 34 that interconnect the heater trace 24 toconnector holes 36. As will be explained in more detail below, theconnector holes 36 are used for attaching an external power source forgenerating a current through the heater trace 24. This, in turn, willgenerate heat that is localized at the bend region 30 to permit foldingor bending of the substrate 22.

[0023]FIG. 2 shows the opposite side of the substrate 22. On theopposite side of the substrate 22, a plurality of conductive traces 38are formed on the first portion 26 and second portion 28 of thesubstrate 22. The conductive traces 38 interconnect a plurality ofelectronic components 40 mounted on the substrate 22. The electroniccomponents 40 may be any electronic component or device that can bemounted to a printed circuit board such as, for example, a battery, acapacitor, a resistor, a semiconductor chip, a diode, an inductor, and acoil. The conductive traces 38 are also electrically attached to theconnector holes 36 for attachment to external connector pins (notshown). It will be appreciated by one of ordinary skill in the art, withthe benefit of this disclosure, that the exact layout of the conductivetraces 38 and the number of components 40 is not critical, but that thepresent invention can be used for many types of electrical circuitswithout departing from the spirit and scope of the present invention.

[0024] In the bend region 30 of the substrate 22, a series of connectingtraces 42 are formed to interconnect the conductive traces 38 formed onthe first portion 26 and second portion 28 of the substrate 22.Advantageously, the conductive traces 38 and the connecting traces 42may be formed and covered with a rigid solder mask. That is, solder maskhaving an elongation of less than 10 percent. Typically flexiblecircuits require the use of flexible solder mask, i.e. solder maskhaving an elongation of up to 30 percent to account for tensile stressintroduced in flexing the circuit board. Flexible solder mask, however,is substantially more expensive than rigid solder mask.

[0025] In one embodiment, the bend region 30 has a first side 44(FIG. 1) and a second side 46 (FIG. 2). The heater trace 24 ispreferably formed on the first side 44 of the bend region 30 and theconnecting traces 42 are formed on the second side 46 of the bend region30. This is more fully illustrated in FIG. 3. FIG. 3 is across-sectional view of the bend region 30 of the substrate 22. Thesubstrate 22 has at least one layer 48 of glass weave impregnated withresin. On each side of the layer 48, there is a layer 50 of epoxy. Thisepoxy layer 50 may be resin-coated copper (RCC) that is commerciallyavailable with FR4 substrate material. Formed on the first side 44 ofthe bend region 30 is the heater trace 24. Formed on the second side 46of the bend region 30 is a connecting trace 42. The connecting trace 42may be electrically connected to the conductive traces 38 of the firstportion 26 and the second portion 28 of the substrate 22 by vias 52. Thecross-section further illustrates an outer solder mask layer 54 onopposite sides of the circuit board 20. It is preferred that the soldermask layers 54 not extend into the bend region 30.

[0026] Referring now to FIGS. 4A and 4B, an exemplary electronic controlunit 60 having a flexible circuit board 20 is shown. FIG. 4A is anexploded perspective view of the electronic control unit 60 inaccordance with an embodiment of the present invention. FIG. 4B is aside view of a portion of the electronic control unit 60 showing thebend region 30 of the substrate 22 in one embodiment. The electroniccontrol unit 60 has a rigidizer 62 and the flexible circuit board 20.The rigidizer 62 has a first rigidizer portion 66 and a second rigidizerportion 68 interconnected by a connecting rigidizer portion 70.

[0027] As explained above, the flexible circuit board 20 has asubstantially rigid substrate 22 and an electrical resistance heatertrace 24. The substantially rigid substrate 22 has a first portion 26and a second portion 28 that is interconnected by a bend region 30. Inthis embodiment, the first portion 26 of the substrate 22 is attached tothe first rigidizer portion 66. The second portion 28 of the substrate22 is attached to the second rigidizer portion 68. In one embodiment,the attachment may be secured by an adhesive such as a pressuresensitive adhesive (PSA) tape or film. In another embodiment, theadhesive may be a heat curable, liquid adhesive that is screen printedon the first rigidizer portion 66 and the second rigidizer portion 68.Those who are of ordinary skill in the art, having the benefit of thisdisclosure, will realize that there are many techniques for securing thesubstrate 22 to the rigidizer 62, such as mechanical fasteners such asscrews or other adhesive laminates that may be placed on the rigidizer62, that may be used herein without departing from the spirit and scopeof the present invention.

[0028] The rigidizer 62 surrounds the flexible circuit board 20 and isdesigned to shield the substrate 22 and electronic components 40 fromelectrical charge which can damage the substrate and components.Accordingly, the rigidizer 62 may also include a sidewall portion 64 tofurther interconnect the first rigidizer portion 66 and the secondrigidizer portion 68. The sidewall portion 64 may be attached to thefirst rigidizer portion 66 and the second rigidizer portion 68 by asolder or weld. The attachment may also be secured through mechanicalfasteners such as screws or an adhesive. The rigidizer 62 may alsoprovide mechanical support and conductively dissipate heat for thesubstrate 22.

[0029] Preferably, the rigidizer 62 is manufactured from materials thatare rigid enough to provide a rigid mechanical support for the flexiblecircuit board 20. In automobile applications, the rigidizer 62 shouldalso be designed to shield the electronic components 40 from heat,water, chemicals, and electrostatic charge. Suitable materials for therigidizer 62 are aluminum, steel, engineering grade plastic, magnesium,and zinc or any material that is resistant to chemicals and elementscommonly found in an automobile. Preferably, the rigidizer 62 is furthermanufactured from thermally conductive materials and conductivelytransfers heat by components 40 during operation of the electroniccontrol unit 60. However, those of ordinary skill in the art willrealize that electronic control unit 60 may be used in low poweroperations where thermal issues, and the thermal conductivity ofrigidizer 62 may be of minor importance.

[0030] The rigidizer 62 also includes a plurality of connector holes 76that are arranged in the same manner to match the connector holes 36 inthe substrate 22. The rigidizer 62 may further include mounting holes 78to attach a connector housing (not shown). A plurality of connector pinson an external connector (not shown) may extend through the connectorholes 76 in the rigidizer 62 and into the connector holes 36 of thesubstrate 22 to provide an electrical interface to the circuitryresiding on the substrate 22.

[0031] As explained above, substrate 22 is preferably made from arelatively rigid material such as FR4 or a multi-layer polyimidematerial. The heater trace 24 is formed on the bend region 30 of thesubstrate 22. When an electric current is applied to the heater trace24, the bend region 30 will be locally heated by the heater trace 24.This will allow the first portion 26 of the substrate 22 to be foldedrelative to the second portion 28 of the substrate 22.

[0032] In one embodiment, as shown in FIG. 4B, an electric current maybe applied to the heater trace 24 by connecting the heater trace 24 to apower supply 80 and a ground 82. The power supply 80 and the ground 82may be electrically connected to the heater trace 24 through theconnector holes 36 in the substrate 22. Alternatively, the ground 82 canbe a case ground. The connector holes 36 are interconnected to theheater trace 24 by connector traces 32 and 34 as illustrated in theplane view in FIG. 1. The amount of voltage and current supplied to theheater trace 24 will vary on the layout of the heater trace 24. However,it was found that a voltage of 13 volts and 0.8 amps was adequate toprovide a sufficient electric current through the heater trace 24 tolocally heat and allow bending of a substrate 22 at the bend region 30.The substrate 22 in that case was made of an FR4 material and the heatertrace 24 was about 10 mils wide and about 25 mils apart in pitch (fromcenter to center) in a serpentine shape. The serpentine heater trace 24extended uniformly across the entire bend region 30.

[0033] It will be appreciated that the present invention provides thesignificant advantage of locally heating the bend region 30 of thesubstrate 22. It limits the heating to only the part of the substratethat needs to be bent. It uses a conductive heater trace 24 thatrequires only a small amount of energy to heat the bend region 30,thereby reducing manufacturing costs. The added cost of the trace isminimal compared to the amount of energy required to bake the entireboard in an oven for bending. Additionally, the time constant forheating is very short compared to baking the entire board in an oven.Little soak time is needed to heat the bend region 30 because thethermal mass is much smaller.

[0034] It also has been found by the inventor that localized heatingpermits bending at a much lower temperature than that required inprevious methods. For example, an FR4 material may have a glasstransition temperature of about 150° C. It has been known to heat theentire board in an oven to within 110° C. of the glass transition priorto bending the substrate. However, it has been found, through thepresent invention, that a local heating of bend region 30 by the heatertrace 24 would permit bending at temperatures of 85° C. This wasachieved by using an electric current of 13 volts at 0.8 amps throughthe heater trace 24 described above. An electric current of 17 volts at0.9 amps through the heater trace 24 yielded about 125° C. in the bendregion 30. Accordingly, less energy is used to permit bending of thesubstrate 22.

[0035] It is anticipated that the heater trace 24 need only be connectedto the power source 80 and ground 82 when bending of the substrate 22 isrequired. The power source 80 and ground 82 may be disconnected afterthe electronic control unit 60 is in the shape shown in FIG. 4A.

[0036] After an electric current passes through the heater trace 24, andthe substrate 22 is folded at the bend region 30, the actual bend formedat the bend region 30 may take a variety of shapes. For example, inFIGS. 4A and 4B, there is a bend 72 that is in a W-shape. The W-shapepermits very compact folding that can be of particular importance forautomotive and other industrial applications. The W-shape bend 72 may beformed when the first rigidizer portion 66, the second rigidizer portion68 and the connecting rigidizer portion 70 are all relatively flat. Asexplained above, the first portion 26 of the substrate 22 is attached tothe first rigidizer portion 66 and the second portion 28 of thesubstrate 22 is attached to the second rigidizer portion 68. Thisattachment may be done when the first rigidizer portion 66 and thesecond rigidizer portion 68 are laying flat relative to each other inthe same plane. The bend region 30 of the substrate 22 is not attachedto the connecting rigidizer portion 70. When the first portion 26 of thesubstrate 22 is folded relative to the second portion 28 of thesubstrate 22, this will form a W-shape bend 72 as shown in FIGS. 4A and4B.

[0037] Referring now to FIGS. 5A and 5B, another type of bend is shownin the bend region 30 of the substrate 22. FIG. 5A is an explodedperspective view of an electronic control unit 160 in accordance withanother embodiment of the present invention. FIG. 5B is a side view of aportion of the electronic control unit 160. The electronic control unit160 has a rigidizer 162 and the flexible circuit board 20. The rigidizer162 has a first rigidizer portion 166 and a second rigidizer portion 168interconnected by a connecting rigidizer portion 170. Here, however, theconnecting rigidizer portion 170 has a depression 174. If the rigidizeris made of sheet aluminum, the depression 174 may be formed in theconnecting rigidizer portion 170 by a stamping process.

[0038] The presence of the depression 174 in the connecting rigidizerportion 170 permits the bend 172 in the bend region 30 of the substrate22 to be U-shaped. The depression 174 in the connecting rigidizerportion 170 is used for housing at least a portion of the bend 172 inthe bend region 30 of the substrate 22. This U-shape also permits verycompact folding that can be of particular importance for automotive andother industrial applications. The U-shape bend 172 may be formed afterthe first portion 26 of the substrate 22 is attached to the firstrigidizer portion 166 and the second portion 28 of the substrate 22 isattached to the second rigidizer portion 168. The bend region 30 of thesubstrate 22 is not physically attached to the connecting rigidizerportion 170 or the depression 174. When the first portion 26 of thesubstrate 22 is folded relative to the second portion 28 of thesubstrate 22, this will form the U-shape bend 172 as shown in FIGS. 5Aand 5B.

[0039]FIG. 6 is a flow diagram or method of assembling an electroniccontrol unit that includes a rigidizer and a flexible circuit board inaccordance with an embodiment of the present invention. As shown inblock 202, the method includes the step of providing a substrate 22. Thesubstrate 22 may be of a substantially rigid material such as FR4 orother organic based substrate material. In one embodiment, the providedsubstrate 22 has a first portion 26, a second portion 28 and a bendregion 30. The bend region 30 interconnects the first portion 26 and thesecond portion 28.

[0040] The process proceeds in block 204 where the method furtherincludes forming a heater trace 24 on the substrate 22. In particular,the heater trace 24 is formed on the bend region 30 of the substrate 22.In one embodiment, the heater trace 24 is formed in a serpentine shapethat is uniform along the entire bend region 30. The heater trace 24should be of sufficient width and length to provide heat to the bendregion 30. In one embodiment, an adequate width of the heater trace 24was found to be about 10 mils and the spacing between each serpentinetrace was about 25 mils in pitch. The heater trace 24 may be depositedand etched to one side of the substrate 22. An insulation layer such asepoxy should cover the heater trace to provide insulation from otherstructures such as the rigidizer.

[0041] The process may proceed to block 206 where connector traces 38are formed on the substrate 22. In one embodiment, the connector traces38 are formed on a side opposite that of the side used when forming theheater trace 24 on the substrate 22. However, in more complex circuits,both sides of the substrate 22 may have connector traces 38 tointerconnect components 40 on the substrate 22. The connector traces 38may be formed on the substrate 22 by masking and etching techniques(chemical, mechanical or optical). Along with the connector traces 38,conductive pads may be located on the substrate 22 for attaching aplurality of electronic components 40.

[0042] As stated in block 208, the method may further include attachinga plurality of electronic components 40 to the substrate 22. In oneembodiment, the components 40 are surface mountable components that maybe auto-placed on the substrate 22 through a pick-and-place machine.

[0043] The process may then proceed to block 210 where a rigidizer 62 isprovided. The rigidizer 62 is essentially the housing for the substrate22 or flexible circuit board 20. In one embodiment, the rigidizer 62 hasa first rigidizer portion 66, a second rigidizer portion 68, and aconnecting rigidizer portion 70. The connecting rigidizer portion 70interconnects the first rigidizer portion 66 and the second rigidizerportion 68. In one embodiment, the rigidizer 62 is made of a metallicmaterial such as sheet aluminum. Additionally, in an alternativeembodiment, the connecting rigidizer portion may be stamped with adepression 174.

[0044] At block 212, the method may further include attaching thesubstrate 22 to the rigidizer 62. Here, the first portion 26 of thesubstrate 22 may be attached to the first rigidizer portion 66 and thesecond portion 28 of the substrate 22 may be attached to the secondrigidizer portion 68. In one embodiment, the attachment may be securedby an adhesive such as a pressure sensitive adhesive (PSA) tape or film.The attachment may also be by a liquid adhesive that is screen printedon the rigidizer and heat curable. Further, the attachment may also beby mechanical techniques or other adhesive laminates.

[0045] The process proceeds in block 214 where there is a step ofapplying an electrical current to the heater trace 24. The electricalcurrent may be provided by an external power source 80. The currentshould be sufficient to generate heat in the heater trace 24 so that itheats the bend portion 30 of the substrate 22. When heated, the bendportion 30 of the substrate 22 becomes more soft and flexible. Thisallows the substrate 22 to be folded or otherwise bended into shape.

[0046] In block 216, the process further includes folding the firstportion of the substrate relative to the second portion of the substrateto form at least one bend in the bend region of the substrate 22. Asshown above, the bend may take a variety of shapes but in one embodimentthe bend 72 is W-shaped and in another embodiment the bend 172 isU-shaped.

[0047] What has been described is a device and method for increasing theflexibility of a circuit board through localized heating. The device andmethod permits relatively small radii bends so that the circuit boardmay be more compactly folded than what could otherwise be accomplishedusing FR4 or similar relatively brittle materials. The alternative is touse single layer polyimide or similar flexible materials at asubstantial cost penalty. The present invention also permits localizedheating of only the area that requires the bend. This reduces the amountof energy needed for assembly costs that can be of particular interestin high volume production.

[0048] The above description of the present invention is intended to beexemplary only and is not intended to limit the scope of any patentissuing from this application. For example, the present discussion usedan electronic control unit to illustrate the device and method of thepresent invention. The present invention is also applicable to otherapplications that use flexible circuits that need to be confined to asmall area. The present invention is intended to be limited only by thescope and spirit of the following claims.

What is claimed is:
 1. A flexible circuit board comprising: asubstantially rigid substrate having a first portion and a secondportion interconnected by a bend region; and an electrical resistanceheater trace formed on the bend region of the substrate; wherein thefirst portion of the substrate is capable of being folded relative tothe second portion of the substrate to form at least one bend in thebend region when an electric current is applied to the heater trace. 2.The flexible circuit board of claim 1 wherein the substantially rigidsubstrate is made of at least one layer of glass weave impregnated withepoxy resin.
 3. The flexible circuit board of claim 1 wherein the firstportion and the second portion of the substrate are formed withconductive traces for interconnecting a plurality of electroniccomponents, the bend region of the substrate formed with connectingtraces for interconnecting the conductive traces formed on the firstportion of the substrate with the conductive traces formed on the secondportion of the substrate.
 4. The flexible circuit board of claim 3wherein the bend region of the substrate has a first side and a secondside, the heater trace formed on the first side of the bend region andthe connecting traces formed on the second side of the bend region. 5.The flexible circuit board of claim 1 wherein the substrate further hasa connector hole, the heater trace electrically connected to theconnector hole.
 6. The flexible circuit board of claim 1 wherein thebend formed in the bend region is W-shaped.
 7. The flexible circuitboard of claim 1 wherein the heater trace is in a serpentine shape thatextends uniformly across the bend region.
 8. An electronic control unitcomprising: a rigidizer having a first rigidizer portion and a secondrigidizer portion interconnected by a connecting rigidizer portion; asubstantially rigid substrate having a first portion and a secondportion interconnected by a bend region, the first portion of thesubstrate attached to the first rigidizer portion, the second portion ofthe substrate attached to the second rigidizer portion; and anelectrical resistance heater trace formed on the bend region of thesubstrate; wherein the first portion of the substrate is capable ofbeing folded relative to the second portion of the substrate to form atleast one bend in the bend region when an electric current is applied tothe heater trace.
 9. The electronic control unit of claim 8 wherein thesubstantially rigid substrate is made of at least one layer of glassweave impregnated with epoxy resin.
 10. The electronic control unit ofclaim 8 wherein the first portion and the second portion of thesubstrate are formed with conductive traces for interconnecting aplurality of electronic components, the bend region of the substrateformed with connecting traces for interconnecting the conductive tracesformed on the first portion of the substrate with the conductive tracesformed on the second portion of the substrate.
 11. The electroniccontrol unit of claim 10 wherein the bend region of the substrate has afirst side and a second side, the heater trace formed on the first sideof the bend region and the connecting traces formed on the second sideof the bend region.
 12. The electronic control unit of claim 8 whereinthe connecting rigidizer portion has a depression for housing at least aportion of the bend in the bend region of the substrate.
 13. Theelectronic control unit of claim 8 wherein the substrate further has aconnector hole, the heater trace electrically connected to the connectorhole.
 14. The electronic control unit of claim 8 wherein the bend formedin the bend region is W-shaped.
 15. The electronic control unit of claim8 wherein the heater trace is in a serpentine shape that extendsuniformly across the bend region.
 16. A method for assembling anelectronic control unit comprising the steps of: providing asubstantially rigid substrate having a first portion and a secondportion interconnected by a bend region; forming an electricalresistance heater trace on the bend region of the substrate; providing arigidizer having a first rigidizer portion and a second rigidizerportion interconnected by a connecting rigidizer portion; attaching thefirst portion of the substrate to the first rigidizer portion; attachingthe second portion of the substrate to the second rigidizer portion;applying an electrical current to the electrical resistance heatertrace; folding the first portion of the substrate relative to the secondportion of the substrate to form at least one bend in the bend region.17. The method of claim 16 wherein the substantially rigid substrate ismade of at least one layer of glass weave impregnated with epoxy resin.18. The method of claim 16 including the additional steps of: formingconductive traces on the first portion and the second portion of thesubstrate for interconnecting a plurality of electronic components; andforming connecting traces on the bend region of the substrate forinterconnecting the conductive traces formed on the first portion of thesubstrate with the conductive traces formed on the second portion of thesubstrate.
 19. The method of claim 16 wherein the step of providing arigidizer further includes a rigidizer having a depression in theconnecting rigidizer portion for housing at least a portion of the bendin the bend region of the substrate.
 20. The method of claim 16 whereinthe step of attaching the first portion of the substrate to the firstrigidizer portion and the step of attaching the second portion of thesubstrate to the second rigidizer portion includes the use of a pressuresensitive adhesive.